The present invention relates to the drying of moist fibrous webs. More particularly, the present invention relates to simultaneously drying and pattern imprinting both sides of moist fibrous webs in the production of sanitary tissue, paper towels, etc.
Moist paper webs are commonly dried by being pressed against the surface of a heated roll. This can be accomplished by passing the moist web through the nip between a pressing roll and the heated roll, with the rolls being compressed together under a sufficient load to provide good thermal contact between the web and the surface of the heated roll. However, since moist webs typically lack sufficient structural integrity to support themselves, they are usually overlaid on a carrier fabric for support. Accordingly, both the carrier fabric and the web are normally drawn through the nip between the pressing and heated rolls. In addition to providing support, the carrier fabric also provides a permeable surface allowing for the absorption and escape of any liquid or vapor as the web passes through the nip.
A common apparatus for drying paper webs in this manner is the so-called Yankee Dryer. In the Yankee Dryer, the web is firmly pressed against a steam heated cylinder by a pressing roll.
In addition, other, more efficient heated-roll web-drying apparatuses are disclosed in U.S. Pat. No. 4,324,613. In these apparatuses, the external surface of the heated roll is heated directly by an externally mounted heat source such as, for example, gas burners positioned near the roll surface. By heating the external surface of the roll in this manner, the surface of the heated roll can be heated to a temperature sufficient to cause a very rapid and violent generation of steam at the interface between the roll and the moist web during the pressing step. The steam thus formed tends to pass straight through the web, carrying with it any free water remaining in the cavities between the fibers of the web. Accordingly, the rapid generation of steam greatly enhances the drying effect of the apparatus by physically removing liquid water from the web.
In addition to modifications of such heated-roll drying apparatuses aimed at increasing the drying efficiency thereof, modifications of the carrier fabric have been proposed as a means for increasing the softness, bulk and absorbency of the dried web. For example, in European Patent Application No. 109307, published Jun. 23, 1984, a carrier felt is disclosed having yarn strands which form knuckles adjacent to and protruding above the web contacting side of the felt. Consequently, as the paper web and carrier felt pass through the nip, the knuckles densify those portions of the web between the knuckles of the felt and the dryer surface to a greater degree than those portions of the web that are being pressed against the surface of the dryer by the felt facing located between adjacent imprinting yarn strands. Reportedly, by using such a felt, the less dense portions of the web produced tend, on average, to be thicker than the thickness of a web produced by using a comparable smooth-surfaced felt, which accounts for the increased softness, bulk and absorbency characteristics of the resulting web.
However, such a method of imprinting a pattern into the surface of a web is less than optimal since it is only capable of imprinting a pattern into one surface of the web, and results in a web having one smooth planar surface and one nonplanar patterned surface. Moreover, such a method for imprinting a pattern into only one surface of the web is inadequate to prevent the destruction, during the drying operation, of patterns of raised and recessed areas created in both surfaces of the moist web during web formation.
Accordingly, it is an object of the present invention to provide an efficient method for imprinting a pattern on both sides of a web while contemporaneously drying the web by contact with the surface of a heated roll. Additionally, it is also an object of the invention to provide an efficient method for drying a web without damaging patterns of raised and recessed areas created in the surfaces of the moist web during web formation.
The foregoing objectives have been satisfied by the method of the invention described in detail below.